Kolkata and Mumbai are most prone to flooding in the wake of climate change induced disasters

Kolkata is ranked among the top 10 cities in the
world in terms of exposure to flooding under climate change projections (IPCC
2012;UN-HABITAT 2010b; World Bank 2011a). The elevation of Kolkata city and the
metropolitan area surrounding the city ranges from 1.5–11 meters above sea
level (World Bank 2011a). Kolkata is projected to be exposed to increasing
precipitation, storm surges, and sea-level rise under climate change scenarios.
Roughly a third of the total population of 15.5 million (2010 data; UN-HABITAT
2010) live in slums, which significantly increases the vulnerability of the
population to these risk factors. Furthermore, 15 percent of the population
live by the Hooghly River and are highly exposed to flooding. Another factor
adding to the vulnerability of Kolkata is unplanned and unregulated
urbanization; infrastructure development is insufficient and cannot keep pace
with current urbanization rates (World Bank 2011a).

Coastal cities vulnerable to flooding

A recent study by the World Bank (2011a)106 on
urban flooding as a consequence of climate-change finds that a 100-year return period
storm will result in doubling the area flooded by a depth of 0.5–0.75m (i.e.
high threat level) under the A1F1 climate change scenario (this scenario
considers a projected sea-level rise of 27 cm and a 16 percent increase in
precipitation by 2050). This excludes Kolkata city, which is analyzed
separately, as the city has sewerage networks in place; these sewerage networks
are essentially absent in the peri-urban areas surrounding the city. According to
the projections presented in Chapter 5 on “Regional Sea-level Rise”, the
sea-level rise in Mumbai and Kolkata is expected to reach 25 cm by the
2030s–40s. In Kolkata city, with a population of approximately five million and
a population density almost three times higher than the metropolitan area (the
city has a population density of 23,149 persons per km² while the metropolitan
area has a population density of only 7,950 people per km²), a flood depth of
more than 0.25 meters is expected to affect 41 percent of the city area and
about 47 percent of the population in 2050 compared to 39 percent of the city
area and 45 percent of the population under the baseline scenario (World Bank
2011a).

40% of Kolkata's urban population can be under threat in case of a 25 cm rise in sea - level

In terms of damages in Kolkata city only, which
accounts for an area of around 185 km² (the metropolitan area surrounding the city
is about 1,851 km²) the World Bank (2011a) study estimates the additional
climate-change-related damages from a 100-year return-period flood to be $790
million in 2050 (including damages to residential buildings and other property,
income losses, losses in the commercial, industrial, and health care sectors,
and damages to roads and the transportation and electricity infrastructures).
Due to data constraints, both total damages and the additional losses caused by
increased flooding as a consequence of climate change should be viewed as
lower-bound estimates (World Bank 2011a).

“Regional Sea-level Rise”, in Kolkata is expected to reach 25 cm by the
2030s–40s.

Given that sea-level rise is projected to
increase beyond 25 cm to 50 cm by 2075 (and 75 cm by 2100) in the lower warming
scenario of 2°C, these risks are likely to continue to grow with climate
change.

Scale
of Flooding Risks with Warming, and Sea-level Rise

With a few exceptions, most of the studies
reviewed here do not examine how flooding risks change with different levels of
climate change and/or sea-level rise. In specific locations, this very much
depends on local topographies and geography; on a broader regional and global
scale, however, the literature shows that river flooding risks are quite
strongly related to the projected level of warming. Recent work by Arnell et al
(2013) reinforces earlier work, showing that the proportion of the population
prone to river flooding increases rapidly with higher levels of warming.
Globally about twice as many people are predicted to be flood prone in 2100 in
a 4°C world compared to a 2°C scenario. Arnell and Gosling (2013) find that
increases in flooding risk are particularly large over South Asia by the 2050s,
both in percentage and absolute terms. Reinforcing this are recent projections
of the consequence of snow reductions in the Himalayan region: increasing
frequency of extremely low snow years causes extremely high northern hemisphere
winter/ spring runoff increasing flood risks (Diffenbaugh et al 2012).

When sea levels rise, where will the floods waters run off?

Part excerpts taken from

Turn Down the
Heat

June
2013

A
Report for the World Bank by

the
Potsdam Institute for Climate Impact Research and Climate Analytics